In the past years, plastics have become commercially vital materials used in a wide range of applications. Particularly, plastics may be applied as transparent articles, as translucent articles, or to opaque surfaces to provide texture, shine, and durability. Generally, these applications require a high degree of abrasion resistance in addition to other required properties, for example, impact strength, tensile strength, or elongation.
To date, no plastic exists which can be made to possess all desirable properties in the desirable proportions. Therefore, a coatings technology has developed which allows the joining of plastic materials and thereby a joining of their desirable properties. For example, the coatings technology may be used to join a substrate possessing high impact resistance, tensile strength, non-opacity, and elongation resistance, such as for example, a polycarbonate substrate, with an outer surface coating possessing high abrasion resistance, and low susceptibility to attack by solvents such as for example, an etherified aminoplast derived coating to produce a single article possessing the structural strength of the polycarbonates and the resistance to surface attack of the etherified aminoplast derivatives.
Coatings suitable for use on plastic substrates must also possess other secondary properties which render them useable. For example, a coating applied for abrasion resistance must also be compatible with the substrate, have good adhesion to the substrate and preferably, be weather resistant, thermoformable, and solvent resistant. It is often in secondary properties that a particular coating may be judged unsuitable for use in any particular application. Therefore, it is in the secondary properties that much effort for improvement is directed.
U.S. Pat. No. 4,197,392, assigned to the same assignee as the present invention, discloses a UV resistant transparent coating for plastic materials containing a melamine, a polyol, and a benzophenone. This coating has both weather resistance and thermoformability, however a greater degree of these two properties is desirable.
U.S. Pat. No. 4,096,125 discloses water-borne polycaprolactone coatings wherein polycaprolactone polyols are reacted with a polycarboxylic anhydride and subsequently with hexamethoxymethylmelamine. This coating has insufficient weather resistance and thermoformability.
U.S. Pat. No. 3,994,851 discloses a water-borne abrasion resistant coatings containing the reaction products of hexamethoxymethylmelamine and polycaprolactone polyols having a molecular weight less than about 850. Those coatings lack sufficient levels of adhesion and thermoformability.
It is therefore, an object of the present invention to provide protective coating compositions which may be applied to plastic substrates, particularly polycarbonate substrates to impart abrasion resistance, solvent resistance and weather resistance.
It is another object of the present invention to provide a protective coating for plastic substrates, particularly polycarbonate substrates, which will continue to adhere and weather well subsequent to a thermoforming process.
It is another object of the present invention to provide a method for protecting sensitive thermoplastic sheet from abrasion and solvent attack, even after prolonged weathering or thermoforming.
It is yet another object of the present invention to provide an abrasion resistant and solvent resistant coated thermoplastic article which will withstand thermoforming and prolonged weathering.
Various other objects and advantages of this invention will become apparent to those skilled in the art from the accompanying description and disclosure.